The use of lasers for the transmission of information in communication systems is known. Such use has typically been limited to amplitude or phase modulated systems that in use often attain a speed of several megabytes.
Laser systems in current use for communications (e.g., solid state pumped 1550 nm lasers with 150 mW output available for fiber optic communication system from Laser Power Corp., San Diego, Calif.) are typically amplitude modulated because of certain inherent limitations in a laser's ability to change frequency. Lasers, in fact, are often limited to a single frequency or a narrow range of frequencies. The tendency of a laser to operate within narrow ranges is inherent in the resonant cavity used in the generation of laser signals.
A resonant cavity of a laser is designed to amplify optical signals of a desired frequency and attenuate signals of an undesired frequency. The cavity amplifies desired frequencies through use of a laser cavity dimensioned in one-quarter wavelength increments. The closer the cavity dimensions are to a desired tolerance, the narrower the range of frequencies within which the laser will operate (the linewidth). The narrower the linewidth, the less inherent amplitude and/or phase noise will be transmitted in a laser signal. Further, the narrower the linewidth, the more power is focused into a desired center frequency.
While gas lasers have been developed with extremely narrow linewidths, solid state lasers do not perform nearly as well. Distributed feedback (DFB) semiconductor lasers, in fact, are known to have relatively wide linewidths.
As the junction current of a DFB laser is changed (or the cavity temperature changes), the operating frequency of the laser also changes. Thus, the linewidth may also vary. Static variations in the inside cavity dimensions may cause the cavity to inherently resonate at a number of frequencies. Variations in the junction current may cause a center frequency to shift (i.e., hop) from one resonant regime to another. Changes in cavity dimensions caused by temperature changes will have a similar effect.
Because of their inherent low cost and reliability, DFB lasers have an enormous potential in laser communication systems. Consequently, a need exists for a method of controlling the linewidth of DFB lasers and for efficiently modulating such devices.